genes are composed of nucleic acids (usually dna)
DESCRIPTION
Genes are composed of nucleic acids (usually DNA). Pneumococcus can be transformed from an avirulent to a virulent strain DNA is the transforming principle DNA in bacteriophage particles appears in the progeny, but very little protein does. Structures of nucleic acids. Nucleotides - PowerPoint PPT PresentationTRANSCRIPT
Genes are composed of nucleic acids (usually DNA)
• Pneumococcus can be transformed from an avirulent to a virulent strain
• DNA is the transforming principle
• DNA in bacteriophage particles appears in the progeny, but very little protein does.
Structures of nucleic acids
Nucleotides
DNA structures
Sedimentation and Electrophoresis
A simple view of DNA
AGCCTCGCATTCGGAGCGTA
Nucleotides
• 3 components to nucleotides:
– Purine or pyrimidine base
– Ribose (RNA) or 2-deoxyribose (DNA) sugar
– Phosphate
• Base + sugar = Nucleoside
• Base + sugar + phosphate = Nucleotide
Types of bases in nucleotides
Pyrimidine
N
N
H
O
NH2
Cytosine
N
HN
H
O
O
Uracil
N
HN
H
O
O
CH3
Thymine
Amino- Keto-
Nucleotides: purine bases
N
N N
N
H
HN
N N
N
H
NH2
H2N
O
Adenine Guanine
6-aminopurine A keto-purine
Bases are attached to C1’ of the sugar via an N-glycosidic bond
N
N N
N
NH2
O
OH
CH2HO
Adenosine
3'
5'1'
2’-deoxy- , a nucleoside
Phosphate is attached to C5’ of the sugar
1st phosphate is a phosphoester, others are attached as phosphoanhydrides.
-O-P-
O=
O
NTP is
-O-P-
O=
O -O-P-O
O=
O
Obase
OH OHphosphoanhydride
phosphoester
-
Structure of a dinucleotide
The 3’ C of one nucleotide is linked to the 5’ C of the next nucleotide in a phosphodiester linkage.
O
O
CH2O
3'
POO
O
HN
NN
N
NH2
O
OH
CH2OPO
O
H
N
N
O
NH2
5' cytidylyladenylateor5'pCpA
5'
3'
5'
Nucleic acids are linear chains of nucleotides
• The 3’ C of one nucleotide is linked to the 5’ C of the next nucleotide.
• The linkage is by a phosphoester.
• The chain has an orientation defined by the sugar-phosphage backbone.
• One terminal nucleotide has a “free” 5’ end, and the other has a “free” 3’ end.
• Thus we designate orientation by 5’ to 3’.
More on orientation of chains of nucleic acids
• 5’ ACTG 3’ is different from 3’ ACTG 5’
• Unless specified otherwise, a chain is written with the 5’ end on the left and the 3’ end on the right.
• When complementary strands in DNA are written, usually the top strand is written 5’ to 3’, left to right, and the bottom strand is written 3’ to 5’, left to right.
5’ GATTCGTACCG
3’ CTAAGCATGGC
Basics of DNA structure
• 2 complementary strands of nucleic acids
• Complementarity is based on H-bonding between– Keto bases with amino bases– Pyrimidines with purines
• A pairs with T (or U)
• G pairs with C
• The complementary strands are antiparallel.
• The complementary strands are coiled around each other.
Duplex DNA• Two strands coil around each other.• Right-handed coils (B form and A.• Coils form major and minor grooves.• Strands have opposite polarity (antiparallel).• Opposing bases in strands are complementary.• Different edges of paired bases are exposed in major and
minor grooves.• Sugar-phosphate backbone is on the outside, bases on the
inside– B-form DNA: base pairs are close to center of long axis of the duplex.– A-form nucleic acids: base pairs stack away from long axis.
Implications of complementarity
• One chain (strand) of DNA can serve as the template for synthesis of the complementary chain.
• DNA replication: sequence of nucleotides in one chain of the duplex determines the sequence of nucleotides in the other chain.
• Transcription: sequence of nucleotides in one chain of the duplex determines the sequence of nucleotides in mRNA or its precursor.
Base pairs in DNA
NN N
N
O
O
N H
H
N
HH
H
deoxy-ribose deoxy-
ribose
N
N N
N
NN N
N
O
NH
H
H
O CH3
deoxy-ribose deoxy-
ribose
Guanine : Cytosine Adenine : Thymine
Major groove
Minor groove
Major groove
Minor groove
Major types of duplex nucleic acid structures
• B form– Most common form of DNA– Right handed DNA-DNA helix– Base pairs stack close to DNA central axis
• A form– right handed RNA-DNA and RNA-RNA helix– Base pairs stack away from the central axis
• Z form DNA– Repeating purines and pyrimidines– Left-handed helix–May serve as some regulatory signal in cells
Forms of nucleic acid duplexes
B-form DNA A-form (e.g. duplex RNA)
Z DNA
Helical parameters for B, A and Z nucleic acids
B A Zhelix sense RH RH LHbp per turn 10 11 12vertical rise per bp 3.4 2.56 3.7 Angstromsrotation per bp +36 +33 -30 degreeshelical diameter 19 23 18 Angstroms
Hyperchromic shift when DNA is denatured
Native duplex DNA Denatured, strands are separate
+
denaturation by heat orincreasing pH
renaturation by cooling or lowering pH
lower A260
higher A260
hyperchromic
hypochromic
1.0
1.2
1.4
Tm= melting temperature, midpoint of the transition
Temperature
A260
Factors that affect melting temperature, p. 85
• The melting tempera-ture (Tm) increases as– Increase G+C– Increase ionic strength
(or )
• Tm decreases as– Increase denaturants– Increase number of
mismatches
Tm = 0.41 (% G+C) + 16.6 log M + 81.5 -0.7 (% formamide) -1o (% mismatch)
% G+C
Tm
= 0.06 = 0.21
Electrophoresis to measure SIZE
For molecules of the same shape, logM is inversely proportional to d.
For molecules of the same size, more compact forms, such as supercoiled DNA, moves faster than more extended forms, such as linear DNA.
+
-
Size markersDNA samples
Example of gel electrophoresis
Alpha-globingenePCR product217 bp
Markers
100
200300400 base pairs